Prostaglandins (PGs), especially PGE2, are abundantly produced by osteoblasts and are critical for anabolic responses in bone to mechanical loading and for normal bone fracture healing. The committed step in PG synthesis is catalyzed by cyclooxygenase (COX). COX has two isoforms in osteoblasts, constitutively expressed COX-1 and inducible COX-2. COX-2 is induced by cytokines, growth factors and hormones and is the principal regulator of PG synthesis in bone. We will study the regulation and role of COX-2 expression in bone both in vivo and in vitro. We hypothesize that a major role for COX-2 is to modulate effects of regulators of bone turnover that induce COX-2, such as parathyroid hormone (PTH). Absence of COX-2 may have little effect on bone mass in unstressed mice, as a result of balanced effects of COX-2 on bone resorption and formation and/or compensatory COX-1 expression. We will examine bone turnover in mice with disruption of the COX-2 gene (COX-2-/-) and their wild-type littermates (COX-2+/+), with and without 2 weeks of PTH injection. We will also examine effects of disrupting one COX-1 allele (COX-1+/-) on the COX-2-/- skeletal phenotype. PGs can be anabolic, stimulating osteoblastic differentiation and bone formation, or catabolic, stimulating osteoclast formation and bone resorption. We hypothesize that COX-2 expression stimulates osteoblastic differentiation by regulating osteoblast proliferation and survival and stimulates osteoclast differentiation by promoting expression of receptor activator of nuclear factor (NF)kappaB ligand (RANKL) in osteoblasts. We will study these hypotheses in cultured primary calvarial osteoblasts from COX-2+/+ and -/- mice. We will also assess the role of PGE2 in mediating effects of COX-2. Finally, we will examine the transcriptional regulation of COX-2 expression by PTH in vivo and in vitro. COX-2 expression in bone cells is largely regulated at the level of transcription. For the in vivo studies, we will use mice transgenic for portions of the murine COX-2 promoter fused to a luciferase reporter (Pluc mice). For the in vitro studies, we will use MC3T3-E1 osteoblastic cells stably transfected with wild-type Pluc constructs and Pluc constructs carrying site-directed mutations of potential cis-acting sites. A better understanding of the regulation and function of COX-2 may lead to novel therapies for preventing bone loss, improving bone gain, and accelerating fracture healing.